1. Field of the Invention
The present invention relates to a method of and an apparatus for reproducing on a monitor, such as a CRT (cathode-ray tube), an LCD (liquid crystal display) or a PDP (plasma display panel), colors of the output of a color printer as of electrophotography, and also to a computer-readable recording medium storing a color-printer-output reproducing program.
2. Description of the Related Art
Generally, because a color printer and a monitor are quite different from each other in color expressing method, it is essential to convert color signals (color signals as of YMCK) for the color printer into those (color signals as of RGB) for the monitor in order to express the same color as the output of the color printer. It has been customary that such conversion is performed by measuring a given color chip, which is previously set for the color printer, to obtain a standard space value, such as CIEXYZ(1931) or CIELAB, and then utilizing a correlation between the standard space value and the color signal for the monitor.
The conventional method for reproducing the color printer output on a monitor is exemplified by the following:
(a) Method using Neugebauer""s formula:
Neugebauer""s formula is to be used in determining values of the individual color components of R, G, B from the dot-area percentage (value to be designated by a color signal) of four color pigments of yellow (Y), magenta (M), cyan (C) and black (K) In this method, a weighted average for values of color measurement of 16 kinds of color chips is calculated using the products of values of arbitrary color components of Y, M, C, K as weighting coefficients. The method is popular in the printing field and is known to have encountered with a problem that, especially in color printing, it is impossible to match the result of real printing with the color to be reproduced on a monitor, as the method is simple but is suitable to use only in ideal condition. Many modifications of the method have been proposed to eliminate the above-mentioned problem; but no successful result has been found.
(b) Method disclosed in Japanese Patent Laid-Open Publications Nos. HEI 10-73487, 10-73488 and 10-73489:
This method is to be used for printed entity (thing printed in ink having transparency). In this method, using spectral reflectivities for 352 color chips, a target spectral reflectivity is calculated by reducing the number of dimensions successively like first from a 4-dimensional color solid (4-dimensional color space) to a 3-dimensional color solid (3-dimensional color space) and then from the 3-dimensional color solid to a 2-dimensional color plane, and vice versa. The spectral reflectivity calculating method for the side (side of only black) representing a black gradation in a color space or color plane is different from that for remaining sides. As a result, color signals for printed entity are converted into color signals for CRT and are displayed on CRT.
However the foregoing conventional methods (a) and (b) have the following problems:
(a) With method using Neugebauer""s formula,
(a-1) Although the result of real printing and the color reproduced on a monitor would tend to match with each other if dither or dot-areas are random, it has turned out that they generally do not match with each other as mentioned above.
(a-2) Some modifications of this formula have been proposed, but in any such modification, no match appears between the real printing result and the color reproduced on a monitor likewise in the case of (1-1).
(a-3) As described later with reference to FIG. 2, an optical phenomenon occurs at the peripheral portion of a substrate supporting the color pigment and cannot be reproduced on paper and a uniform-all-over color chip; this optical phenomenon is not considered in Neugebauer""s formula. This is the reason why the real printing result and the color monitor reproduced on a monitor do not match with each other.
(b) With the methods disclosed in Japanese Patent Laid-Open Publications Nos. HEI 10-73487, 10-73488 and 10-73489:
(b-1) Any of these conventional methods is only for use with printing entity (printed in ink having a transparency). Whereas in color printing using electrophotography, it is customary to use powder toner, which does not allow light to pass. In such event, assuming that a color pigment is laid over another color pigment, the lower color pigment becomes invisible. Therefore attempt to apply the conventional method in which only transparent ink is considered would be, as a matter of fact, unsuccessful in reproducing color on a monitor.
(b-2) In electrophotography in particular, if the dot-area percentage of black is 100%, namely, if black is printed in uniform-all-over manner, the spectral reflectivity would be roughly constant irrespective of the value of any other color pigment. As a result, in the conventional method in which only transparent ink is thought, it is impossible to estimate a spectral reflectivity when the dot-area percentage of black is 100%.
(b-3) Generally in a color printer, even an image printed in the same color signal in the same page would result in different colors, depending on the local printing region, due to remarkable variation of color. This color deviation is called xe2x80x9cin-page errorxe2x80x9d. Because the in-page error cannot be negligible, it would be difficult to realize highly detailed estimation. For increase estimation precision of spectral reflectivity, it has been cherished to measure the to-be-deviated color so that a desired spectral reflectivity is forcibly calculated for the color. Namely, it has been cherished to designate a particular spectral reflectivity for the combination of specified color signals (dot-area percentages); but these conventional methods fail to disclose or teach any technique of realizing the foregoing cherished achievements.
With the foregoing problems in view, it is a first object of the present invention to provide a method of matching the real output color of a color printer with the color to be reproduced on a monitor by estimating spectral reflectivities precisely with consideration of an optical phenomenon peculiar to the color printer output.
A second object of the present invention is to provide an apparatus for carrying out the above-mentioned method.
A third object of the present invention is to provide a computer-readable recording medium storing a color-printer-output reproducing program.
The above-mentioned first object is accomplished by a method of reproducing on a monitor the output of a color printer which makes color printing using color pigments of n colors (n is a natural number of 2 or larger) including black in combination, with black being printed as a final stage of the color printing, the method comprising the steps of:
calculating at least one coefficient to be used in calculating a particular spectral reflectivity corresponding to an arbitrary dot-area percentage, based on spectral reflectivities measured for a plurality of previously designated color chips (including more than 2 standard color chips of different dot-area percentages on each side) on nxc2x72xe2x88x921 sides of an n-dimensional color solid;
calculating the particular spectral reflectivity corresponding to the arbitrary dot-area percentage based on the coefficient calculated in the coefficient calculating step and the plural spectral reflectivities measured for the plural color chips; and
converting the particular spectral reflectivity calculated in the spectral reflectivity calculating step into a color signal for the monitor;
wherein in the coefficient calculating step, more than two coefficients representing color characteristics of the output of the color printer and respectively relating to the plural spectral reflectivities of the more than two standards color chips are calculated, based on the plural spectral reflectivities measured for the plural color chips, for one of the nxc2x72nxe2x88x921 sides which is neither a side where the dot-area percentage of black is 100% nor a side representing a black graduation, and
wherein in the spectral reflectivity calculating step, a particular spectral reflectivity corresponding to arbitrary dot-area percentage for the side which is neither the side where the dot-area percentage of black is 100% nor the side representing a black gradation is calculated based on the more than two coefficients calculated in the coefficient calculating step, and a particular spectral reflectivity corresponding to an arbitrary dot-area percentage on the side where the dot-area percentage of black is 100% is calculated based on the plural spectral reflectivities measured for the plural color chips.
The method may further comprise a step of discriminating whether or not color information to be reproduced on the monitor is that for the color printer, based on the plural spectral reflectivities measured for the plural color chips. In the discriminating step, if the result of the discrimination is positive in the discriminating step, the coefficient calculating step, the spectral reflectivity calculating step and the converting step are carried out.
In the discriminating step, a standard deviation of the plural spectral reflectivities measured for the plural color chips on the side where the dot-area percentage of black is 100% may be calculated and then the calculated standard deviation may be compared with a predetermined value, whereupon if the calculated standard deviation is equal to or less than the predetermined value, the color information to be reproduced on the monitor may be judged to be for the color printer.
Alternatively, in the coefficient calculating step, the more than two coefficients may be calculated by the least squares method based on the plural spectral reflectivities measured for the plural color chips. As another alternative, in the coefficient calculating step, two coefficients "xgr", xcex7 representing dotgains, which relate respectively to the spectral reflectivities of two reference color chips when the corresponding dot-area percentages are 0% and 100%, may be calculated for every side except the side where the dot-area percentage is 100% and the side representing a black gradation, based on the plural spectral reflectivities measured for the plural color chips. And in the spectral reflectivity calculating step, the two coefficients "xgr", xcex7 calculated in the coefficient calculating step may be used in calculating the particular spectral reflectivity corresponding to the arbitrary dot-area percentage.
As still another alternative, in the coefficient calculating step, the plural spectral reflectivities measured for the plural color chips on the side representing a black gradation may be averaged throughout at least part of a visible wavelength range, and two coefficients "xgr", xcex7 representing dotgains are calculated based on an average value of the plural spectral reflectivities as the result of the averaging. And in the spectral reflectivity calculating step, the particular spectral reflectivity corresponding to the arbitrary dot-area percentage on the side representing a black gradation may be calculated, based on the two coefficients "xgr", xcex7 calculated in the coefficient calculating step.
Further, in the spectral reflectivity calculating step, a constant value to be calculated or determined based on the plural spectral reflectivities measured for the plural color chips may be outputted as the particular spectral reflectivity corresponding to the arbitrary dot-area percentage on the side where the dot-area percentage is 100%.
In the spectral reflectivity calculating step, firstly an (nxe2x88x921) -dimensional color solid, in which the dot-area percentage of an n-th color pigment to be printed by the color printer is constant, may be calculated from the n-dimensional color solid, secondly an (nxe2x88x922)-dimensional color solid, in which the dot-area percentage of an n-th color pigment to be printed by the color printer is constant, may be calculated from the (nxe2x88x921)-dimensional color solid, thirdly a 2-dimensional color plane, in which the dot-area percentage of 3rd to n-th color pigments is constant, may be calculated by repeating a reduction process of the same number of dimensions as described above, and finally, a spectral reflectivity when the color pigments of n colors set to arbitrary dot-area percentages are combined, may be calculated, using the two coefficients "xgr", xcex7, from the 2-dimensional color plane.
At that time, in order to designate a specified value for the spectral reflectivity when the color pigments of n colors set to specified dot-area percentages are combined, a weighting coefficient may be calculated based on the inverse number of a distance between a color signal imparting the specified dot-area percentage and a color signal imparting the arbitrary dot-area percentage, then a value obtained by multiplying the specified value with the weighting coefficient may be added to the finally calculated spectral reflectivity. And if the distance is 0 (zero), the specified value may be outputted as the finally calculated spectral reflectivity.
The above-mentioned second object is accomplished by an apparatus for reproducing on a monitor the output of a color printer which makes color printing using color pigments of n colors (n is a natural number of 2 or larger) including black in combination with black being printed as a final stage of the printing, the method comprising:
a coefficient calculating section for calculating at least one coefficient to be used in calculating a particular spectral reflectivity corresponding to an arbitrary dot-area percentage, based on spectral reflectivities measured for a plurality of previously designated color chips (including more than 2 reference color chips of different dot-area percentages on each side) on nxc2x72nxe2x88x921 sides of an n-dimensional color solid;
a spectral reflectivity calculating section for calculating the particular spectral reflectivity corresponding to the arbitrary dot-area percentage based on the coefficient calculated in the coefficient calculating section and the plural spectral reflectivities measured for the plural color chips; and
a converting section for converting the particular spectral reflectivity calculated in the spectral reflectivity calculating section into a color signal for the monitor;
the coefficient calculating section being operable to calculate more than two coefficients representing color characteristics of the output of the color printer and respectively relating to the plural spectral reflectivities of the more than two reference color chips, based on the plural spectral reflectivities measured for the plural color chips, for one of the nxc2x72nxe2x88x921 sides which is neither a side where the dot-area percentage of black is 100% nor a side representing a black gradation;
the spectral reflectivity calculating section being operable to calculate a particular spectral reflectivity corresponding to arbitrary dot-area percentage for the side which is neither the side where the dot-area percentage of black is 100% nor the side representing a black gradation, based on the more than two coefficients calculated in the coefficient calculating section, and being also operable to calculate a particular spectral reflectivity corresponding to an arbitrary dot-area percentage on the side where the dot-area percentage of black is 100% is calculated based on the plural spectral reflectivities measured for the plural color chips.
The apparatus may further comprise a discriminating section for discriminating whether or not color information to be reproduced on the monitor is that for the color printer, based on the plural spectral reflectivities measured for the plural color chips. If the result of the discrimination is positive in the discriminating section, the operations of the coefficient calculating section, the spectral reflectivity calculating section and the converting section are carried out.
The above-mentioned third object is accomplished by a computer-readable recording medium storing a color-printer-output reproducing program which instructs a computer to execute a function of reproducing on a monitor the output of a color printer which makes color printing using color pigments of n colors (n is a natural number of 2 or larger) including black in combination with black being printed as a final stage of the printing, the color-printer-output reproducing program instructing the computer to function as the following:
a coefficient calculating section for calculating at least one coefficient to be used in calculating a particular spectral reflectivity corresponding to an arbitrary dot-area percentage, based on spectral reflectivities measured for a plurality of previously designated color chips (including more than 2 reference color chips of different dot-area percentages on each side) on nxc2x72xe2x88x921 sides of an n-dimensional color solid;
a spectral reflectivity calculating section for calculating the particular spectral reflectivity corresponding to the arbitrary dot-area percentage based on the coefficient calculated in the coefficient calculating section and the plural spectral reflectivities measured for the plural color chips; and
converting section for converting the particular spectral reflectivity calculated in the spectral reflectivity calculating section into a color signal for the monitor;
the coefficient calculating section being operable to calculate more than two coefficients representing color characteristics of the output of the color printer and respectively relating to the plural spectral reflectivities of the more than two reference color chips, based on the plural spectral reflectivities measured for the plural color chips, for one of the nxc2x72xe2x88x921 sides which is neither a side where the dot-area percentage of black is 100% nor a side representing a black gradation;
the spectral reflectivity calculating section being operable to calculate a particular spectral reflectivity corresponding to arbitrary dot-area percentage for the side which is neither the side where the dot-area percentage of black is 100% nor the side representing a black gradation, based on the more than two coefficients calculated in the coefficient calculating section, and being also operable to calculate a particular spectral reflectivity corresponding to an arbitrary dot-area percentage on the side where the dot-area percentage of black is 100% is calculated based on the plural spectral reflectivities measured for the plural color chips.
At that time, the color-printer-output reproducing program further instructs the computer to function as a discriminating section for discriminating whether or not color information to be reproduced on the monitor is that for the color printer, based on the plural spectral reflectivities measured for the plural color chips, and wherein if the result of the discrimination is positive in the discriminating section, the color-printer-output reproducing program instructs the computer to function as the coefficient calculating section, the spectral reflectivity calculating section and the converting section.
According to the color-printer-output reproducing method and apparatus, and the computer-readable recording medium of the present invention, since the spectral reflectivity can be estimated precisely with consideration of an optical phenomenon peculiar to the color printer output, it is possible to match the real color of the color printer output and the color to be reproduced on the monitor.